A muffled sound is heard by a person who is wearing earphones because his or her external auditory canals are closed by the earphones. This phenomenon is caused by a resonance in the external auditory canals that are closed by the earphones.
JP-A-2007-110536 discloses a technique relating to a configuration of noise canceling headphones. In the noise canceling headphones, multiple filter characteristics having different frequency pass-bands are stored in advance for noise cancellation. An arbitrary one of the multiple filter characteristics is selected by an external operation and set as a filter characteristic of a digital filter.
JP-A-H5-252598 discloses a technique of selecting a proper head-related transfer function (HRTF) by causing a user to hear many sample sounds with each of his or her left and right ears. JP-A-H8-111899 discloses a technique that relates to hardware for solving a problem of JP-A-H5-252598.
However, in the function of selecting one of multiple settings, no highly convenient method has been disclosed which utilizes a relationship between characteristics of the external auditory canal(s).